In recent years, a secondary battery, which can be repeatedly charged and discharged, has been widely used as an energy source for wireless mobile devices. In addition, the secondary battery has attracted considerable attention as an energy source for electric vehicles and hybrid electric vehicles, which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles that use fossil fuels. As a result, kinds of applications using the secondary battery are being increased owing to advantages of the secondary battery, and hereafter the secondary battery is expected to be applied to more applications and products than now.
Based on the construction of electrodes and an electrolyte, the secondary battery may be classified as a lithium ion battery, a lithium ion polymer battery, or a lithium polymer battery. In particular, the lithium ion polymer battery has been increasingly used because the lithium ion polymer battery has a low possibility of electrolyte leakage and can be easily manufactured. Based on the shape of a battery case, the secondary battery may also be classified as a cylindrical battery having an electrode assembly mounted in a cylindrical metal can, a prismatic battery having an electrode assembly mounted in a prismatic metal can, or a pouch-shaped battery having an electrode assembly mounted in a pouch-shaped case made of an aluminum laminate sheet. The electrode assembly mounted in the battery case functions as a power generating element, including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, which can be charged and discharged. The electrode assembly may be classified as a jelly-roll type electrode assembly configured to have a structure in which a long sheet type positive electrode and a long sheet type negative electrode, to which active materials are applied, are wound in the state in which a separator is disposed between the positive electrode and the negative electrode or a stacked type electrode assembly configured to have a structure in which a plurality of positive electrodes having a predetermined size and a plurality of negative electrodes having a predetermined size are sequentially stacked in the state in which separators are disposed respectively between the positive electrodes and the negative electrodes.
Much interest is currently focused on the increase in size of a case and the decrease in thickness of a material based on the increase in capacity of batteries. As a result, a pouch-shaped battery cell, configured to have a structure in which a stacked type or stacked/folded type electrode assembly is mounted in a pouch-shaped battery case made of an aluminum laminate sheet, have been increasingly used because the cost of manufacturing the pouch-shaped battery is low, the pouch-shaped battery is lightweight, and it is easy to modify the shape of the pouch-shaped battery.
FIG. 1 is a perspective view typically showing a conventional representative pouch-shaped battery.
Referring to FIG. 1, the pouch-shaped battery, denoted by reference numeral 10, is configured to have a structure in which two electrode leads 11 and 12 protrude from the upper end and the lower end of a battery body 13, respectively, such that the electrode leads 11 and 12 are opposite to each other. The electrode leads 11 and 12 are provided at the respective ends of the battery body 13 such that the electrode leads 11 and 12 are located on the same axis while the electrode leads 11 and 12 are located at the middle of the battery body 13.
A sheathing member 14 includes upper and lower sheathing parts. That is, the sheathing member 14 is a two-unit member. In the state in which an electrode assembly is mounted in a receiving part 13 which is defined between the upper and lower sheathing parts of the sheathing member 14, opposite sides 14b and upper and lower ends 14a and 14c, which are contact regions of the upper and lower sheathing parts of the sheathing member 14, are bonded to each other. The sheathing member 14 is configured to have a laminate structure of a resin layer, a metal foil layer, and a resin layer. Consequently, it is possible to bond the opposite sides 14b and the upper and lower ends 14a and 14c of the upper and lower sheathing parts of the sheathing member 14, which are in contact with each other, to each other by applying heat and pressure to the opposite sides 14b and the upper and lower ends 14a and 14c of the upper and lower sheathing parts of the sheathing member 14 so as to weld the resin layers thereof to each other.
In the pouch-shaped battery, a large amount of gas may be generated in the battery case and the pressure in the battery case may be increased under the extreme conditions, such as high temperature, high voltage, and high current, with the result that the battery may be ignited or exploded. In order to solve this problem, it is necessary to discharge the gas out of the battery case.
During charge and discharge of the pouch-shaped battery, high pressure is induced in the sealed battery case, with the result that the battery case may swell. In this case, the sealed portion of the battery case may be broken, whereby gas may be discharged out of the battery case. In this case, however, it is not possible to check the portion of the battery case from which gas has been discharged through the broken region of the sealed portion.
Meanwhile, a gas discharge member may be attached to a portion of the battery case in order to discharge gas of the battery case. In this case, however, the cost and process of manufacturing the gas discharge member are increased.